To meet increasingly demanding performance requirements, navigation technology development must be rapid and innovative, relying on a system-of-systems vision that involves a multi-orbit space component – with low Earth orbit (LEO) PNT becoming a critical factor.
MICHEL MONNERAT, HANAA AL BITAR, THALES ALENIA SPACE
Positioning, Navigation and Timing (PNT) services currently rely almost exclusively on GNSS in medium Earth orbit (MEO). The first GPS system (NAVSTAR), developed in the 1980s by the U.S. Department of Defense, was followed by several other constellations such as Galileo, GLONASS, BeiDou, and QZSS, providing end users with a vast array of navigation signals capable of supporting a wide range of applications.
The market adoption of such capabilities went tremendously fast. Today, PNT is an integral part of our day-to-day lives, impacting critical infrastructure, asset tracking, agriculture and Industry 4.0, as well as intelligent transport systems. The evolution of the application ecosystem is pushing PNT systems to the limits of their capacity, generating highly dynamic research efforts to meet growing needs as effectively and swiftly as possible. Low Earth orbit (LEO) PNT has become a critical factor in making that happen.
In this article, we analyze new trends in PNT application demands, the evolution of PNT systems and the rise of LEO PNT, putting user needs at the center of future technological advancements.
The New PNT Drivers
The use of GPS by the general public, benefiting from the development of digital boards, marked the first milestone in the adoption of satellite navigation in daily life.
In the early 2000s, the technical requirements related to emergency calls (E911 in the United States and E112 in Europe) played a truly catalytic role in the adoption of GNSS in the market, providing precise and fast location of distress calls made from mobile phones. The E911 mandate required an accuracy target of 50 meters at 67% despite the challenges of integrating a GNSS receiver into a mobile phone.
Today, satellite navigation is commonly and routinely used in both professional and public frames, while still maintaining its original strategic purpose for governmental and military applications.
To this end, several countries have developed their own global satellite navigation systems, such as the European Union’s Galileo, Russia’s GLONASS, and China’s BeiDou.
PNT’s success and constant integration into the economy, in turn, has steered us to more stringent requirements. The race for new critical requirements has led to an unprecedented need for evolution in performance, resilience, integrity, ubiquity and connectivity.
Enhanced performance: This includes accuracy, ubiquitous continuity and availability, time to first fix and extended coverage.
Robustness: PNT must protect against outliers and anomalies like interference, spoofing and multipath.
Safety: This is driven by integrity and continuity objectives where integrity is a measure of trust of the computed position.
Connectivity: For many applications, positioning shall be associated to the capacity of exchanging the positions of the user with a third party application or just to exchange position-related data (location based services).
Ubiquity: Operations anytime, everywhere, in diverse and harsh environments is more and more mandatory.
Quantifying these performance elements, or related Key Performance Indicators (KPIs), in a unified manner for different application domains is very challenging, as the operational domains, and their purposes, vary from one application to another, and within the same application.
Several sources of such KPI implementations can be found in the state of the art solutions being developed like [1-3].
Why Satellites Are Still the Cornerstone
Satellite positioning remains the backbone of all these techniques and is the most widely used among numerous positioning technologies. What makes satellites stand apart from all the existing and emerging positioning technologies?
First, satellites offer key technical differentiators that are essential for many applications:
• Worldwide coverage
• Consistent performance
• An absolute reference
• Resilience to meteorological and space weather conditions
Secondly, the satellite is part of a global approach that can withstand any local event or disaster, with independent operation from the number of users.
Toward a Multi-Orbit Constellation?
It is important to highlight that some PNT requirements for the applications considered, when taken alone, may be reachable with a medium Earth orbit constellation by adjusting the signal and constellation geometry. However, the combined need for enhanced performance, increased robustness, AND integrity for safety is the challenge. To meet these challenges, we need a
multi-orbit vision.
Although very efficient in their design, constellations in MEO, around 20,000 km, suffer from a number of limitations, often the downside of their advantages. The first one is the relatively low dynamic range of the satellites, which means their geometry varies slowly, making multipaths virtually stationary or an evolution of measurements that is not favorable for quick ambiguity resolution when aiming to rapidly converge to a high-precision position. Another limiting factor is the low power of signals received on the ground (around -155dBW), which makes them vulnerable to jamming, whether intended or not, and spoofing.
Finally, the medium-orbit layer represents the secure backbone required by its strategic aspect, which requires a complex design. Service evolution cycles can hinder the dynamism of application requirements.
The use of multiple GNSS constellations and multiple frequencies is one answer to the question of resilience and availability. However, it is not a sufficient answer to counter the stationarity of multipath effect or the need for low power consumption. This approach is, therefore, often complemented by the combined use of additional sensors at user level (inertial sensors, LiDAR, cameras, etc.). However, hybridization is often not enough, either. The multiplication of sensors still suffers from problems of reliability (integrity), coherence and the ability to have an absolute reference in a masked or blurred environment. To meet the need for high accuracy in a fast convergence time, availability and resilience while providing an absolute reference, solutions are moving toward adding a LEO component to MEO systems.
It is important to highlight the European Space Agency’s (ESA) LEO-PNT In-Orbit Demonstrator within the FutureNAV program [4], which aims to demonstrate how to provide resilience and augment Galileo with a low-Earth orbit layer; the BeiDou program’s statement along the same lines from initiatives in China; and Geely Geespace’s initiative to provide a PNT low-Earth orbit constellation to support its own autonomous car development, showing just how central PNT is to autonomous car concerns.
LEO PNT: A Game-Changer
The deployment of PNT systems in LEO has been studied for many years, both as dedicated constellations and onboard payloads [5,6]. The immediate advantages of LEO PNT lie in a favorable link budget, due to its proximity to the Earth’s surface, but also in a greater dynamic range offering natural multipath filtering and a rapid phase ambiguity resolution capability useful for enhanced performances, mainly in terms of accuracy and time-to-first-fix.
However, low Earth orbit requires a higher number of satellites than medium Earth orbit for an equivalent number of visible satellites. Typically, to ensure four satellites are visible from any point on the Earth’s surface, a constellation of about 300 satellites is needed at an altitude of 600 km. Until now, the cost of such a constellation to guarantee real-time positioning of about 300 satellites in LEO compared to 30 in MEO has been prohibitive. There are several reasons for this.
The first is an isolated constellation in LEO requires a dense ground segment and precise clocks on board the satellites. These clocks have a volume and mass that require large satellites. This first limitation is lifted as soon as we consider a layer in low orbit that relies on the layer in medium orbit that carries the precise clocks. The same applies to orbit calculation which, instead of requiring a dense ground segment, also can be based on measurements from MEO satellite signals.
As the link budget is also favorable because of the low altitude of the orbits, LEO PNT satellites do not require a high-power chain onboard for the same power received on the ground.
As a result, small, quasi-autonomous satellites can be considered, and in a New Space approach, micro-satellite based. LEO also benefits from a favorable radiation balance compared with MEO, enabling the use of commercial off the shelf (COTS) components. However, these COTS components require appropriate implementation and specific know-how to achieve the required level of performance and reliability (radiation protection, redundancy scheme, on-board power management, reconfiguration strategy, etc.). Recently, systems such as Kinéis [7] have demonstrated the ability of these small satellites to achieve levels of reliability and performance compatible with critical commercial missions. The dynamism of the sector and the proximity of low-Earth orbit have also led to many micro launcher initiatives and opportunities for low-cost launches.
These arguments make a LEO constellation economically viable, even for large constellations. Several ongoing projects such as CentiSpace [8], Xona Space Systems [9], GNSSaS [10], as well as the European Space Agency’s FutureNAV Program [4], are examples of the regained worldwide interest in the LEO PNT concept.
The value of a LEO PNT solution is now well established and is often based on the addition of greater flexibility and degrees of freedom, as well as the ability to evolve rapidly:
Increased accuracy and rapid convergence: This is made possible by faster resolution of carrier phase ambiguity in a Precise Point Positioning (PPP) solution.
Increased robustness: LEO PNT provides alternative measurements, often with higher received power and frequency diversity than GNSS MEO systems, in the event of localized GNSS interference or jamming.
Improved availability: LEO PNT provides a diversity of signals (SiS) and frequencies that helps improve availability in urban or indoor environments.
Enhanced flexibility and scalability: Offering shorter development and evolution cycles, LEO PNT is natively highly scalable, as the constellation can be deployed and optimized according to application needs.
Two-way connectivity: The altitude is ideal for an uplink path that consumes very little energy for terminals. It also offers greater capacity for disseminating information by regionalizing transmission.
The LEO PNT implementation perspectives vary today according to three distinct axes:
1. Dedicated LEO PNT constellations.
Such constellations offer a dedicated, defined signal that’s optimized for navigation. This type of constellation offers frequency diversity while remaining in the low L, S or C band for user segment and usage compatibility.
2. Constellations of opportunity. These are constellations in low orbit, so the mission does not natively include navigation. Navigation signals are added opportunistically. This type of approach offers frequency diversity, but the accuracy is naturally degraded compared to what’s obtained with a signal and a system geometry designed for navigation. The geometry of onboard antenna patterns is often ill-suited to navigation, not to mention atomic clocks.
3. Merged communication and navigation constellations. This is the case with telecommunications constellations with signals that take into account the need to provide a navigation service in addition to communication services. The trade-off seems attractive from an economic point of view, but the performances are not comparable to those obtained with dedicated constellations. Often, the frequencies used are high, which makes the user segment more complex (more robust to interference but not suitable for all use cases). The business model may also be different in that access to the navigation resource is linked to access to the main mission and potentially subject to a charge.
The 5G Non-Terrestrial Networks (NTN), and the work currently being carried out by 3GPP, are the most relevant examples of the search for a fusion between communication and navigation missions. Joint optimization remains a major complexity, which suggests it will be difficult to achieve the same level of accuracy, availability, coverage and suitability for all types of users, as can be obtained with a dedicated LEO PNT constellation. Reversely, a dedicated LEO PNT constellation operating in the same frequency band as the telecommunication system could provide a major assistance to the telecommunication function by assisting the terminals to connect to the communication services.
The attractiveness of LEO PNT is illustrated in Table 1, providing a view of the main LEO PNT systems being developed. These figures and, more generally speaking, the LEO constellations are still under constant evolution. More details can be found in [11].
A Need for Hybridization and Reliability
Although LEO PNT offers the perspective of a significant improvement in the provision of an absolute position, it must be considered in the frame of a hybrid PNT solutions context, involving MEO, LEO, terrestrial and non-terrestrial telecom networks, as well as user sensors.
Reliability is also key for adoption for safety critical applications like intelligent transport. It is driven by integrity and continuity targets.
The concept of positioning integrity was originally developed for civil aviation applications. New safety-critical autonomous transport applications, such as cars, drones and trains, require a major adaptation of this heritage. In such environments (urban canyons, interference, etc.), integrity must be shared between the space system and the user segment.
Global and regional threats are addressed at system level. This is done through the definition of a new GNSS high accuracy and integrity service. Local effects, on the other hand, are addressed at the level of the user’s onboard unit [12].
Conclusion
GNSS-based PNT technologies are now the backbone of a multitude of innovative applications. These systems have made considerable progress in recent years, particularly since the arrival of Galileo, which offers new, more powerful signals and services such as decimetric accuracy and anti-blur authentication.
All these new applications have one thing in common: They call for progress in terms of precision, continuity, resilience and security. These needs are driving R&D and producing major developments.
The introduction of a constellation of LEO PNT satellites is key to meeting the needs identified, while respecting economic imperatives. To provide a complete response, it must be designed and used in a multi-sensor context.
References
(1) ETSI TR 101 593, Satellite Earth Stations and Systems (SES); Global Navigation Satellite System (GNSS) based location systems; Minimum performance and features
(2) ETSI TR 103 183, Satellite Earth Stations and Systems (SES); Global Navigation Satellite Systems (GNSS) based applications and standardisation needs
(3) 3GPP TR 38.855, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Study on NR positioning support.
(4) https://www.esa.int/Applications/Satellite_navigation/LEO-PNT
(5) P. K. Enge, B. Ferrell, J. Bennett, D. A. Whelan, G. M. Gutt, and D. G. Lawrence, “Orbital Diversity for Satellite Navigation,” Nashville, TN, Sep. 2012.
(6) T. G. Reid, A. M. Neish, T. F. Walter, and P. K. Enge, “Leveraging Commercial Broadband LEO Constellations for Navigating,” Portland, Oregon, Nov. 2016, pp. 2300–2314. doi: 10.33012/2016.14729.
(7) https://www.kineis.com/en/spatial-iot-connectivity/
(8) Y. Long, “The Centispace-1: A LEO Satellite-Based Augmentation System,” 2019.
(9) “LEO Successor to GNSS Comes Knocking,” Jun. 2020. https://insidegnss.com/leo-successor-to-gnss-comes-knocking/(accessed Jan. 26, 2022).
(10) H. Am, A. K, A.-E. O, and M. Mi, “NSSTC capabilities and the GNSSaS satellite project,” Aeronaut. Aerosp. Open Access J., vol. 4, no. 4, pp. 156–159, Nov. 2020, doi: 10.15406/aaoaj.2020.04.00117.
(11) FrontierSI State of Market Report on Low Earth Orbit Positioning Navigation and Timing (LEO PNT)—2024, https://frontiersi.com.au/wp-content/uploads/2025/01/FrontierSI-State-of-Market-Report-LEO-PNT-2024-Edition-v1.1.pdf.
(12) “Integrity Complementing High Accuracy Service via EGNSS for Autonomous Driving—ICHASE project results,” Hanaa Al Bitar, Tristan Planchais, Rami Ali Ahmad, Anne-Marie Tobie, from Thales Alenia Space, Ni Zhu, Miguel Ortiz, from University Gustave Eiffel, Antonella Di Fazio, from FDC, Silvia Porfili, Javier Ostolaza, Gerarda De Pasquale, European Union Agency for the Space Programme.
(13) https://techwireasia.com/2022/06/chinas-geely-first-batch-of-leo-satellites-for-autonomous-vehicles-launched/
(14) https://developingtelecoms.com/telecom-technology/satellite- communications-networks/16165-china-s-geespace-launches-next-batch-of-leosats-for-connected-cars.html
(15) https://www.01net.com/actualites/volkswagen-et-porsche-vont-lancer-leur-propre-constellation-de-satellites-pour-les-voitures-autonomes-2055976.html
Authors
Hanaa Al Bitar is the user segment and augmentations manager at Thales Alenia Space France (TAS-F), Bids and Advanced Projects department. She received her Ph.D. in Radio Navigation signal processing in 2007 form the Ecole Nationale de l’Aviation Civile in France. She joined Thales Alenia Space in 2012. Her activities focused on GNSS receiver signal processing, EGNOS Land Earth Stations (NLES) signal processing and design, and technical management of end to end demonstrators over EGNOS GEO satellites in 2015 and 2017. Since 2023, she has managed the business and technical roadmap for developing highly reliable and precise positioning solutions for autonomous transport applications.
Michel Monnerat is the Director of Bids and Advanced Projects at Thales Alenia Space (TAS-F Navigation Domain). He is a senior expert at Thales Group in Space Navigation System and has more than 50 patents in space systems. He is also Director of the Navigation Advanced projects department in Thales Alenia Space with more than 25 years of experience in space systems development. He has worked on many space-based radar programs within Alcatel Space and was in charge of developing payloads like ARGOS/SARSAT. He has been involved in Galileo and EGNOS since 1998 as system architect, signal designer, performances manager, regulation manager for ITU and Engineering Director. With his new functions within Thales Alenia Space he is in charge of developing innovative upstream and downstream technologies and services in the navigation sector, with a worldwide customers portfolio that’s both civil and military. He is also in charge of developing New Space programs essentially based on constellations of satellites (Internet of things, Earth navigation augmentation) as well as space exploration systems.
